Advances in phase-change materials

A textbook definition of a phase-change material (PCM) is that it releases or absorbs a substantial amount of energy at a phase transition. The phase change of these materials is usually associated with the change of the state of matter, for example, from solid to liquid or from liquid to gas. In many functional applications, however, it is advantageous to exploit solid-to-solid phase change. The best-known PCM is water, which changes its state between liquid and solid at 0 °C and between liquid and gas at 100 °C. The latter case has been employed as the working mechanism of a steam engine, opening the era of the first industrial revolution in human history.

Because of the large amount of energy absorbed or released in the phase change (referred to as latent heat), PCMs were heavily used as heat storage materials in early research but extended for a wide scope of applications later. So far, phase change of a potential practical use has been found widely in versatile materials, such as alloys, ceramics, organics, and emerging two-dimensional (2D) materials. These new PCMs, as well as their unique phase-change properties, have deepened the understanding of phase transition and enabled a lot of novel functional applications in memories, neuromorphic calculation, photonics, actuators, etc. For these reasons, PCMs have become frontiers of applied physics, materials science, and electrical engineering in recent years.

To highlight the advances in PCMs, we organized a Special Topic in the Journal of Applied Physics. The “Phase-Change Materials: Syntheses, Fundamentals, and Applications” Special Topic aimed to provide a timely forum for sharing recent advances in the research field of PCMs. In addition to original research articles, it also contained invited tutorials and perspectives that may lead to new physical insights and technical advances for future research. A variety of PCMs, including inorganic and organic ones, have been reported in this Special Topic. These PCMs are much needed in diverse applications.

One kind of well-investigated PCMs is the family of vanadium oxides that exhibit different phase-change characteristics depending on stoichiometry. Among them, vanadium dioxide (VO₂) is a textbook PCM due to its well-known metal-to-insulator transition (MIT) near room temperature.¹ In a Perspective, Lu et al. introduced a spin-pairing model of the MIT of VO₂, proposed a new way to estimate the transition temperature, and used the method to study the doping and alloying process.² Devthade and Lee presented a Tutorial on the synthesis of VO₂ thin films and dimensionally oriented VO₂ nanostructures,³ which is quite instructive for beginners who work on the synthesis of this famous PCM. Shao et al. reported the regulation of MIT temperatures of VO₂ by progressively changing lattice-mismatch-induced strain on MgF₂ (111) substrates.⁴ The volatile and non-volatile behavior of MIT can also be regulated by tuning oxygen vacancies through annealing.⁵ These works on the controlled synthesis of high-quality VO₂ guarantee excellent properties of materials and high performance of devices. Some intriguing properties, including thermochromic and photochromic characteristics,⁶ negative differential resistance,⁷ and self-regulate current sharing,⁸ were reported in VO₂- or VO_x-based composite films. From the viewpoint of applications, Ma et al. summarized in their Tutorial recent advances in diverse PCMs for actuators, where VO₂ has been a typical and widely used inorganic actuation material owing to its high volumetric work density and good compatibility with current microfabrication processing.⁹ Gong et al. presented a Perspective on three dominant PCMs, VO₂, Ge–Sb–Te (GST, as will be discussed later), and Si, which were used in reconfigurable photonic devices based on the mechanism of tunable refractive index.¹⁰ In addition, theoretical modeling and experimental probing of the MIT of another vanadium oxide, V₃O₅, by Raman spectra and elastic light scattering,¹¹ may arouse more concerns on the family of vanadium oxides.

GST, or ternary Ge–Sb–Te, are a family of phase-change chalcogenide alloys that have been widely used as key components in phase-change memories and successfully realized into products of DVD-RAM and Blue-ray disks on the market. The grain refinement in GST films was reported to lower the power consumption and improve the endurance performance of phase-change memories.¹² Annealing temperature was found to play a role in the structural evolution and the crystallization process of Ge-rich GST alloys.¹³ Several in situ studies, including in situ transition electron microscopy (TEM),¹⁴ x-ray diffraction (XRD),¹⁵ and optical reflectance measurements,¹⁶ provided useful information on crystallization processes of GST films. Modeling and simulations helped understand the crystallization,¹⁷ metal incorporation,¹⁸ and threshold switches¹⁹ in GST or GeTe materials for phase-change applications. In addition, the study of the importance of contacts in phase-change memory devices based on a similar alloy, Cu₂GeTe₃, may inspire people to investigate a wider scope of phase-change chalcogenide materials.²⁰ Besides phase-change memories, GST was also investigated in reconfigurable photonics. With the reconfigurable function, a slow-light device using a photonic crystal waveguide based on Ge₂Sb₂Te₅ was reported.²¹ In a Perspective, Meng et al. presented the progress of GST-based reconfigurable metasurfaces for the application in nonlinear optics, anapole control, beam steering, absorbers, and polaritons.²² This Perspective, together with that of Gong et al.,¹⁰ provides comprehensive understanding of reconfigurable photonics based on GST PCMs.

PCMs have also been actively studied for thermoelectric, thermally conductive, and thermal storage applications. Scott et al. reported that the room temperature thermal conductivity of (Ge₂Sb₂Te₅)_1−xC_x films is a function of carbon concentration and anneal temperature.²³ Valentini et al. developed a PCM-enhanced plaster from natural materials for eco-sustainable thermal energy storage.²⁴ Oshima et al. experimentally measured the melting points and dissociation enthalpies of mixed semiclathrate hydrates, providing guidelines for latent heat storage in cold air conditioning.²⁵ Biswas et al. demonstrated a proof-of-concept oxygen sensor based on Seebeck coefficient change below MIT transition.²⁶ To provide more physics insight, Shamberger et al. numerically studied the dynamics of an oscillating melting-solidification in a slab subject to harmonic heating and convective cooling, correlating the thermophysical properties of a PCM to its transient thermal response.²⁷ Liquid-liquid phase transition is important for the development of smelting, and Tyagunov et al. experimentally determined the liquid–liquid transition in nickel superalloys melts.²⁸ 

Ferroelectric and ferromagnetic materials have been hot topics in physics and materials science for a long time, and some of them are PCMs. Gao et al. presented in a Perspective an overview of the phase transition mechanisms and dopant-dependent phase transition characteristics in lead-free, antiferroelectric AgNbO₃-based ceramics.²⁹ Sharma et al. reported the coexistence of two ferroelectric phases and improved room-temperature multiferroic properties in BiFeCoO₃–PbTiO₃ prepared by a simple sol-gel route.³⁰ By modeling and simulations, Han et al. proposed a multi-antiskyrmion defects-driven skyrmion nucleation mechanism in both the ferromagnetic single layer and synthetic antiferromagnetic trilayers with the spin-polarized current stimuli.³¹ In terms of organic ferroelectric materials, Verma et al. reported an anomalous temperature-dependent crossover behavior from a non-polar phase to an enhancement in ferroelectric and piezoelectric responses in uniaxially stretched polyvinylidene-co-hexafluoropropylene polymer.³² These Perspectives and research articles may be important supplements to current ferroelectric and ferromagnetic studies, particularly for those with phase-change characteristics.

From well-known PCMs, VO₂ and GST, to new types of PCMs, from thermal applications to ferroelectrics, from computational to experimental investigations, the “Phase-Change Materials: Syntheses, Fundamentals, and Applications” Special Topic covers different aspects of PCMs and their applications. Some uncategorized approaches, including an Ising-like model to analyze a spin liquid³³ and an original fitting approach to extract optical constants of thin films,³⁴ may also be applicable for other PCMs and help understand their phase change characteristics. We hope this collection could inspire more discussion and collaborations in the community to push the further development of PCMs and their impactful applications.